Indian physicist takes new view of gravity on lecture tour

28 March 2014

Nobody understands why the cosmological constant is so tiny, but in an exchange lecture tour organised by the IOP and the Indian Physics Association, Prof. Thanu Padmanabhan has been discussing some new theoretical work that might shed light on the problem.

Prof. Padmanabhan, an eminent theoretical physicist at the Inter-University Center for Astronomy and Astrophysics (IUCAA) in Pune, India, visited the UK from 17-27 March to deliver a series of talks - the Homi Bhabha Lectures - under a scheme in which an Indian physicist, and one from Great Britain and Ireland, deliver lectures in each other’s home countries in alternate years.

Audiences at the universities of Portsmouth and Oxford, the Rutherford Appleton Laboratory, Imperial College London, and finally at Queen Mary, University of London, heard talks on one of his three lecture topics: "The Cosmological Constant: its problem and a possible solution", "Gravity: the Emergent Perspective", and "The Accelerating Universe".

Speaking before setting off for the final lecture, he said the tour had gone well. Being asked to give this year's lectures had provided a good opportunity to see colleagues whom he had connected with over many years of visiting the UK, he said. He spent a year as a postdoc at the Institute of Astronomy in Cambridge (1986-7) and was a senior visiting fellow there in May 1993, visiting faculty in May 2001, and its Sackler Distinguished Astronomer in 2002, 2003, 2009 and 2011. He has also been visiting faculty at three universities in the US.

His first degree and MSc were gained at Kerala University in India, and his PhD work was done at the Tata Institute of Fundamental Research (TIFR). He then held various faculty positions at TIFR, before moving to IUCAA in 1992, where he has been the dean of core academic programmes since 1997.

His path towards becoming a physicist began in childhood. He said: "My father was a mathematician by attitude; he was an administrator but he was a good mathematician and it was his hobby. I also had an uncle who was a good mathematician, so from my earliest days I was fascinated by maths and I was way ahead of the school curriculum. When I was around 15 or 16 someone told me that if I did theoretical physics I could have the best of both worlds of physics and maths."

He published a paper on general relativity while still an undergraduate, and his research has focused on quantum theory, cosmology, gravitation and structure formation in the universe. He said: "In the last few years we’ve been able to come up with a completely different way of looking at gravity. It looks as though the equations that describe gravity are similar to those that describe elasticity or fluid mechanics and that gravity is an emergent phenomenon." Obtaining the gravitational field equations from a thermodynamic principle has the potential to solve the cosmological constant problem, he said.

"We have discovered in the last decade or so that the universe is not only expanding but accelerating. The simplest model that will account for all of these observations is the cosmological constant – but the problem is that nobody understands why it has the tiny value that it has. When we looked at this carefully we found that the issue is somewhat deeper - it's actually a problem of gravity. In all other areas of physics we’re only interested in differences in energy, not its absolute value. But with gravity the absolute value of the energy matters and this has been the source of the problem. The new paradigm offers a way around this difficulty."

Experimental proof of such theories would be extremely difficult, he said. "That's why it's important to use the cosmological constant, where we know its experimental value and it is crying out for an explanation." The recent findings on gravitational waves and inflation from the BICEP2 research seemed to show that the energy scale at which inflation took place was in a similar band to that indicated by his theoretical research, but he would like to see the results stabilise before drawing firmer conclusions.

His numerous awards include the Padma Shri in 2007, a medal of honour from the president of India for distinguished service to the country, and the Third World Academy of Sciences Prize in Physics in 2011, for his work on the emergent gravity paradigm. He has undertaken several roles in the Indian National Science Academy, the International Union of Pure and Applied Physics and the International Astronomical Union.

He believes that physics is in a healthy state in India. "The economy is in a growing phase and the governments have been quite kind to science in India. Our contribution is being recognised and physics is fine at the higher level, but we do have a serious problem with science at the school level. Schoolteachers are grossly underpaid and facilities in schools are somewhat limited."

When he was at school, pieces of equipment were precious resources that were kept locked away in case children broke them, and he believes that little has changed. Children have to be given the freedom to tinker with things if they are to learn through experimentation, he said. This is being practised at a sister institute to IUCAA located within its science park - the Muktangan Science Exploratorium - where children come to learn science by doing, he said.

He is impressed by the way UK universities combine teaching and research. "In India, most high-quality research takes place in the research institutes and there is a divergence between teaching and research, but I don't think you should disentangle them." The faculty of IUCAA is only 15-strong but there is an "extended family" of nearly 90 associate members from other universities who visit the centre for about three months a year. "This model is supposed to help the nucleation and growth of astronomy research in the universities. That's happening to a very great extent but I would like to see it grow more."